Filmmaker/actress Sarah Polley poses for a portrait during the 2013 Sundance Film Festival.

For just about every family, there are one or two stories that remain a tangled bramble of half-remembered anecdotes, exaggerated truths, and self-serving myths. Sorting out what actually happened is usually an impossible task, but that is exactly what actress and filmmaker Sarah Polley (Dawn of the Dead, Away From Her) set out to accomplish with her terrific new documentary Stories We Tell, which opens in New York this Friday and then expands out May 17.

When Polley was 11, her mother, actress Diane Elizabeth, passed away from cancer; by 13, her family began joking about how little Polley resembled her father Michael. But it would be another 15 years or so before Polley would start to sort out the shifting truths about her true parentage, a journey that proved to be so transformative that she felt compelled to begin documenting how it all happened on film. The result is a terrifically perceptive, inventive, and honest exploration of family and memory, and how they both shape who we are in ways we often struggle to understand.

In an exclusive clip from the movie below, Polley's siblings and father talk about the moment when those aforementioned jokes began to trickle into the family dinners.

According to Polley, it was surprisingly not difficult to get her siblings and father to agree to talk with her on camera about this tricky family secret.

"I think some of them, in retrospect, had a lot of reservations, but they didn't really tell me about them, they just said okay," Polley tells BuzzFeed. "I think it was their way of being supportive and realizing this was something I obviously needed to do, so I was very grateful that it wasn't harder to convince them."

It didn't hurt, of course, that everyone involved already had a close relationship with Polley, but when it's pointed out to Polley that her family were remarkably open and personable on camera, she just laughs. "Well, I think what helped was interviewing them over such a long period of time," she says. "Each of them I interviewed for a full day — probably about eight-to-ten-hour interviews — so there was a lot of time for people to relax and drop whatever preconceptions they'd had about how they were going to present themselves or what they were going to say and not say. That, I think, helped me get a clearer picture of who they were across on camera."